Vacuum interrupters find application as circuit protection devices in electrical distribution and motor control systems, and comprise a sealed envelope with movable contacts disposed within the envelope for making and breaking electrical continuity. When the contacts are in a closed current carrying position in contact with each other, the contact must carry large currents efficiently with low resistance values. When the contacts are first separated to open the circuit, an arc is struck between the contacts, vaporizing some portion of the contacts, followed by a rapid quenching of the arc when the contacts are fully open, and interruption of the circuit. The contacts must be readily separable, i.e., have an antiweld characteristic so that the operating mechanism need not exert undue force in moving the contacts apart. While some vaporization of the contact material is necessary to sustain the arc, gross erosion of the contacts is to be avoided since this will give rise to high contact resistance when the contacts are closed for current carrying operation.
The selection of contact materials is therefore a very critical aspect in the functioning of the whole vacuum interrupter apparatus. A widely used contact material is a blend of a high-conductivity material such as copper, with a higher melting point refractory material such as chromium or tungsten. There are a variety of metallurgical processes known by which such contacts can be manufactured. U.S. Pat. Nos. 3,960,554 and 4,190,753 teach chromium-copper vacuum interrupter contacts. U.S. Pat. No. 3,818,163 teaches the use of a chromium or cobalt matrix contact material which is infiltrated with copper and silver. U.S. Pat. No. 2,362,007 teaches the use of about 10% chromium, some phosphorus and the remainder copper, while U.S. Pat. No. 2,758,229 describes an electrical current commutator which is approximately 70% to 90% copper and a 10% to 30% total of chromium, lead, nickel, tin, cadmium, and iron. U.S. Pat. No. 4,299,889 discloses a copper-tungsten mixture. A copper-bismuth mixture is discussed in U.S. Pat. No. 3,246,979, while U.S. Pat. No. 4,204,863 teaches contact material made from mixtures of two silver oxides, for example AgCdO plus AgZnO, while U.S. Pat. No. 4,501,941 teaches contacts made from copper, chromium, and aluminum oxide.
Yamanaka et al., in U.S. Pat. No. 4,424,429 teaches conventional contacts which contain 60 wt.% copper, 25 wt.% chromium, and 15% bismuth. These contacts are said to have rough grains of bismuth. The inventors solve this problem by providing contactors containing 60 wt.% copper or silver; 25 wt.% chromium, tungsten, molybdenum, cobalt or iron; 15 wt.% of an oxide additive having a melting point lower than copper (m.p. 1083.degree. C.) or silver (m.p. 961.degree. C.), selected from bismuth oxide (m.p. 820.degree. C.), thallium oxide (m.p. 300.degree. C.), indium oxide (In m.p. 155.degree. C.), antimony oxide (m.p. 655.degree. C.) or tellurium oxide (m.p. 733.degree. C.); and optionally a titanium compound. These components are mixed as dry powders, compressed, and sintered in a non-oxidative atmosphere, in a vacuum or high purity hydrogen furnace at 1000.degree. C. for 2 hours. While this method provides a fine uniform bismuth layer in continuous network form, an even more improved vacuum interrupter contact is desirable.
It is an object of this interrupter to provide a vacuum interrupter contact material which exhibits high current interruption, low weld strengths, low chop currents at a given voltage, low erosion characteristics, and strong bonding of the bismuth component.